Researchers examined data on more than 44,000 drivers in single-vehicle crashes who died between 1999 and 2009. They found that 24.9% tested positive for drugs and 37% had blood-alcohol levels in excess of 0.08, the legal limit. Fifty-eight percent had no alcohol in their systems; 5% had less than 0.08. The data were from a government database on traffic fatalities.

The risks of drunk driving (and walking) are well-established; it is good (though wildly sobering) to see some hard data on drugged driving.

*Before you start stomping up and down about the relative driving risk of marijuana versus alcohol, do read the first few paragraphs of this post.

** One of Romano’s research areas — along with applied economics, epidemiology, traffic safety — is “Latino issues.” I’ve never seen that listed as an academic field; I wonder how many non-Latinos are in it?

Exactly. I’m all for legalizing marijuana, and making “drugged driving” a criminal offense.

Just like when I tell people I’m for legalizing marijuana, and they say “but I don’t want my doctor to operate on me while he’s stoned”. Yeah, neither do I, but I also don’t want him to operate on me drunk, yet there’s not a huge push for a return to prohibition.

I’m wondering how much overlap there is in these numbers, i.e. how many drugged drivers are captured by the drunk driving number as well? Not to suggest anything about the safety of drugged driving, just wondering how to read this for crashes that involved only marijuana.

There’s a problem with only looking at crashes: it’s difficult if not impossible to estimate the effect a cause has just from those cases. Ashworth, Clinton, Meirowitz, and Ramsay (manuscript 2008) provide an excellent example that shows the fallacy:

“The medical examiner of a small city made a limited claim about the likelihood that the apparent association of consuming beverage X with death represents a real effect. This finding is based on the fact that there were 16 deaths in the previous week. All 16 were beverage-X-drinkers; none was a non-drinker…. This sounds good, until we learn that beverage X is water.”

In probabilistic terms, the author seems to confuse Pr(marijuana|crash) with Pr(crash|marijuana). Those are only equal if Pr(crash)=Pr(marijuana), which strikes me as unlikely.

Thank you – this is exactly the point I was thinking of. The study would have to find a control group (or baseline group) of drivers in the same area/same conditions who did NOT crash and see what percent of them also tested positive for marijuana.

I wonder (and I was not able to access the paper to find out myself) about the threshold for testing positive for marijuana. What if the driver tests positive for marijuana, based on their drug use the day before, or the week before? It is my understanding that marijuana use is regularly detected up to a month after use, but the high/impairment rarely lasts that long…

One problem with the study is that marijuana is fat-soluble and stays in the system for several weeks after ingestion, inflating the numbers. If a person crashes a car while sober, he/she would still test positive for the marijuana they smoked several days earlier. Blood alcohol content dissipates from the system much faster.

Don’t get me wrong – I think drugged driving is a problem. I just don’t necessarily buy the figures in the study.

Actually, I would argue the risk of drunk driving is not well established. People who drink and drive are not the same type of people who do not drink and drive. Until you take a random sample of drivers (actually double blind is the best so you would have to give fake alcohol to some and hidden alcohol laced, non-alcoholic beverages to some of the others) and show that those who then drink real alcohol are more accident prone and less safe drivers, the increased accident rate maybe just a filtering and sorting affect of alcohol drinking. The higher rate of accidents may be due to the same personality traits that lead people to drink. Drinkers, even when sober, may just drive more carelessly and aggressively than non-drinking drivers do.

If drinkers are different psychologically from non-drinkers, the baseline to compare the increased accident rate is not against all drivers, but against the drivers with the same psychological profile as drinkers. Auto accidents and reckless driving occur among sober drivers also, but this accident-prone group maybe of the same type as drinkers who drive. The increased accident rate due to alcohol may be much lower than often cited. I have heard it said that the same 20 percent of drivers are responsible for 80 percent of accidents. If true, alcohol and impairment may not be the reason (or the sole reason) for the increase in auto accidents. The same traits that lead one to drink and drive may also lead one to drive more carelessly. It is not an absurd idea, since it is known that a subset of drunks will often act out aggressively and abusively and use alcohol as the excuse, when it known that they will act similarly even when they are sober.

There would be no point in making the study blind as you describe. It’s not a pharmaceutical trial; the placebo effect is irrelevant (it doesn’t matter whether people suck at driving because of the actual effect of alcohol, or because they think they’re drunk). Also, instead of using a control group with random assignment, it would be better to test each driver both drunk and sober. I believe that such a study is underway at my university, using a driving simulator.